1. Field of the Invention
The present invention relates to devices for sensing the presence of specific ions in fluids. In particular, the present invention relates to ion selective electrode (ISE) technology, and methods of making ion selective electrodes.
2. Description of the Prior Art
An ion-selective electrode (ISE) is an electrode which exhibits an electrical response which is a function of concentration of a specific ion contained in a solution which is in contact with the ISE and a reference electrode. Ion selective electrodes operate on the basis of the Nernst principle, which was discovered by W. H. Nernst, a German physicist, in the late 1800's. The Nernst equation defines a logarithmic relationship between the potential of a solution and its ionic concentration. When an ion selective electrode and a reference electrode are exposed to a solution, a potentiometric measurement can be made between the two electrodes which indicate the concentration in the solution of the particular ion to which the ion selective electrode responds.
The Nernst equation can be written as: EQU Y=M log.sub.10 X+B
where X is ion concentration, Y is the output potential, M is the Nernstian slope, and B is a constant.
Most commercially available ion selective electrodes include an internal reference electrode, an electrolyte (in either liquid or gel form) which is in contact with the internal reference electrode, and a membrane which separates the internal reference electrode and the electrolyte from the solution. The membrane is commonly a glass or polymeric membrane in which an electroactive species is incorporated. The particular electroactive species differs depending upon the particular ion to be sensed.
Coated wire electrodes are a type of ion selective electrode in which an electroactive species is incorporated in a thin polymeric film coated directly to a metallic conductor. Coated wire electrodes differ from other ion selective electrodes in that they do not use an electrolyte as an internal reference solution. Although coated wire electrodes offer simplified construction in contrast to other ion selective electrodes, they have not found significant use other than in experimental studies.
Although ion selective electrode (ISE) technology has been known for several decades, its use generally has been limited to laboratories with highly trained technicians making the measurements and interpreting the data. One of the deterrents to the use of ISE systems outside the laboratory has been the necessity for calibration of electrodes to establish their Nernstian slopes (M) in terms of the millivolt output response (Y) of the electrode per decade change in concentration (X). After this is done, a further measurement has to be made in the test solution to assess its concentration. From time to time, the ISE has to be recalibrated since the Nernstian slope can change by several millivolts and its intercept on the Y axis (i.e. the constant B) can shift.
Still another deterrent is that when ISE's are initially put into use or reused after storage, they need to be equilibrated in a suitable solution. This "preconditioning" of an ISE is time-consuming and inconvenient.
In the past, ISE's have typically exhibited significant drift. One of the major causes of this drift in ISE's is capacitive effects which are uncontrolled and therefore "float". This floating or changing capacitance causes drift, error, and the need for standardization and restandardization. The capacitance effects are related to three significant deficiencies in the prior art ion selective electrodes.
First, the spatial relationship of the reference electrode to the sensing electrode is not fixed.
Second, the prior art ISE's typically are constructed in multiple layers over a conductor, and each of these layers have varying characteristics which give varying capacitances and therefore uncontrollable changes in capacitance.
Third, in certain multilayer ISEs having a hydrophylic layer interposed between the sensing electrode and the conducting layer, the capacitance changes continuously with time as the dried hydrophyllic layer changes its state of hydration during the test. There are still other types of electrodes which have various layers are not fixed; and these can be physically deformed as well, causing additional uncontrollable changes in capacitance.